Search

The timing of deposition of fluvial sediments now forming the >7-km-thick McCoy Mountains Formation is one of the key uncertainties in reconstructing the Mesozoic paleogeography of southern California and western Arizona. Ion-microprobe U-Pb geochronologic data for individual zircons from nine sandstones from the McCoy Mountains type section and six associated igneous rocks provide significant new constraints on the tectonic setting and the timing of deposition within the northwest-trending McCoy basin. U-Pb zircon data from a metavolcanic rock of the underlying Dome Rock sequence in the Palen Mountains confirm that the McCoy Mountains Formation was deposited after regional Middle to Late Jurassic arc magmatism. U-Pb zircon data from a Late Cretaceous granodiorite intruding the formation in the Coxcomb Mountains confirm that the formation was deformed and metamorphosed prior to 73.5 [+ or -] 1.3 Ma. Populations of detrital zircons vary systematically with both rock type and stratigraphic height; lithic arkoses predominantly derived from the west have consistently more abundant younger zircons than do litharenite sandstones predominantly derived from the north, and the youngest zircons yield maximum depositional ages that decrease from 116 Ma near the base to 84 Ma near the top of the section. The detrital-zircon data permit a Late Jurassic age for the basal, comparatively quartz-rich sandstone. However, the data further suggest that >90% of the formation was deposited between middle Early and middle Late Cretaceous time. These results are consistent with the hypothesis that most of the McCoy Mountains Formation represents a retroarc foreland basin, deposited behind the active, evolving Cretaceous Cordilleran continental-margin magmatic arc that lay to the west and in the foreland of the actively deforming Cretaceous Maria fold-and-thrust belt. Keywords: California, stratigraphy, tectonics, geochronology, geochemistry.

The Salinian block in the Salinas Valley region of central California consists of arc granitic and metasedimentary rocks (schist of Sierra de Salinas) sandwiched between coeval high-pressure, low-temperature melange belts. U-Pb zircon ages of three granitic plutons from this region range from 88 to 82 Ma, and coexisting biotite yielded [sup.40]Ar/[sup.39]Ar cooling ages of 76-75 Ma. The U-Pb ages from detrital zircons indicate derivation of the protolith of the schist from a 117-81 Ma igneous provenance. Muscovite and biotite [sup.40]Ar/ [sup.39]Ar cooling ages of 72-68 Ma from the nearby schist are distinctly younger than those from the granitic plutons. These data indicate that deposition and metamorphism of the schist occurred after emplacement of adjacent granitic rocks, contradicting the prevailing view that the schist comprises the local framework for the Salinian arc. We propose that the schist of Sierra de Salinas was thrust beneath the Salinian magmatic arc along a Campanian thrust fault that has not been recognized. This hypothesis implies that the Salinian arc originated as a klippe of basement rocks derived from the vicinity of the western Mojave Desert. Thrusting initiated southeastward-migrating Laramide tectonism of a style similar to that which formed the Vincent thrust and the latest Cretaceous and Paleocene Pelona and Orocopia Schists of southern California and southwestern Arizona. Keywords: geochronology, tectonics, California, Salinia.

The Pelona and Orocopia Schists are southern members of a eugeoclinal terrane that structurally underlies a large part of southwestern North America. Ion-microprobe U-Pb ages of >100 detrital zircons from three widely spaced samples of these two units indicate that deposition occurred after 70-80 Ma. Moreover, the distribution of zircon ages, including a significant peak centered ca. 1.7 Ga, implies a major contribution of detritus from the Mojave Desert and Transverse Ranges of southern California. Recrystallization of the schists at depths of 20-35 km occurred within 10-15 m.y. of deposition, which requires underthrusting at minimum rates of approximately 4-12 mm/yr for reasonable thrust dips. Considering that similar processes formed the more northern Rand Schist at somewhat earlier times, our results indicate a southward progression in timing of deformation for the schists' protoliths. This result is at odds with northward migration of deformation implied by the Baja British Columbia hypothesis. The age and provenance of the schist's protoliths are consistent with models that derive the schists from either the Franciscan Complex or Great Valley Group.

Chemistry is sometimes called the central science because scientists from other fields are focusing their studies on the molecular level. Chemists rarely direct papers to other disciplines, but computational chemistry is a multidisciplinary area of study that integrates biology, chemistry, physics, and mathematics. The fields of geology and chemistry were traditionally distinct until geochemistry courses combined them.

The Josephine Mountain intrusion is a Cretaceous calc-alkalic tonalite-granite pluton emplaced at 22 km depth in a continental margin arc. Variable uplift of adjacent terranes in southern California since mid-Cretaceous time allows us to reconstruct the local crustal column and evaluate its role as a contaminant of mantle-derived arc magmas in this region. The parental magma of the intrusion was high-alumina basalt whose isotopic signature (87Sr/86Sr = 0.7087; [[Delta].sup.18]O = 7.5; [[Epsilon].sup.Nd] = -10) cannot have been generated by intracrustal assimilation of known or inferred rock types in the middle or lower crust. Such a signature could have resulted from high-pressure fractionation of primary low-alumina basalt coupled with assimilation of felsic/pelitic lower crust, partial melting of enriched subcontinental mantle followed by high-pressure fractionation, or a combination of these processes. Tonalite of the intrusion was formed by fractionation of the parent magma coupled with assimilation of local felsic wall rocks or by crustal melts similar to slightly younger granite. Assessment of the magnitude of crustal contamination is hampered by uncertainty regarding the existence and role of partial melting of previously enriched subcontinental mantle in generating the parental basaltic magma, leading to concomitant uncertainty in the fraction of new continental crust created by such arc plutonism.

The Snow Lake pendant is underlain by a thick ([is greater than] 1500 m) sequence of predominantly quartzite, marble, and schist that has previously been correlated with miogeoclinal strata of the western Mojave Desert. In this study, U-Pb analyses of detrital zircons from the Snow Lake pendant have been conducted to test for possible correlations with strata of the Cordilleran miogeocline, as well as nearby rocks of the Roberts Mountains allochthon and the Shoo Fly Complex. Zircons from Snow Lake strata yield dominant age groups that strongly support correlation of Snow Lake strata with miogeoclinal strata in the Mojave Desert. This correlation provides additional support for previous suggestions that the Snow Lake pendant was displaced ~400 km northward along the Mojave--Snow Lake fault. Keywords: zircon, Cordillera, Sierra Nevada, miogeocline, U-Pb geochronology.

The Cajon Pass Deep Scientific Drillhole penetrated a minimum of 1 km of Late Cretaceous (?) intermediate plutonic rocks containing magmatic anhydrite, which occurs as both matrix grains and relic microphenocrysts within poikilitic hornblende, plagioclase, and sphene. Phase equilibria of coexisting silicates, oxides, and apatite support petrographic evidence of hypersolidus anhydrite, indicating that intermediate rocks from a mafic-felsic calc-alkalic suite (or suites) crystallized from about 800 to 700 [degrees] C at [P.sub.tot] ~6 kb from hydrous and oxidized, sulfate-saturated andesitic to dacitic magma. The occurrence of anhydrite as inclusions within early crystallizing phases and its association with sulfate-enriched apatite indicates that, despite the potentially rapid destruction of matrix anhydrite by subareal weathering, petrologic evidence of the nature of such volatile-rich magma systems can be retrieved. If found to be widespread by future work, such evidence could be useful in understanding the mechanisms of volatile enrichment in explosive sulfur-rich volcanic systems and the potential relative roles of metasomatized lithospheric mantle sources and shallow-level mixing/ assimilation processes in Cordilleran arc magma systems.